1. Technical Field
The present invention relates to an image processing method for a nuclear medicine image, such as tomograms of the positron emission tomography (abbreviated as PET hereinafter) and the single photon emission computed tomography (abbreviated as SPECT hereinafter), a program and an image processing apparatus for performing the method. More specifically, it relates to a contour extracting method for a tomogram, such as a PET image, a SPECT image and other nuclear medicine images, and a program and an image processing apparatus for performing the method.
2. Related Art
Nuclear medicine images, such as a PET image and a SPECT image, are useful for diagnosis of wide variety of diseases including cardiopathy and cancer. These images are obtained by administering a medicine labeled with a particular radioisotope (referred to as a radioactive medicine hereinafter) to a patient, detecting gamma rays emitted from the medicine with a dedicated camera, and reconstructing an image. The nuclear medicine image is superior to the other images used for diagnosis not only in disease specificity and sensitivity but also in availability of information on the function of the lesion.
As described above, the nuclear medicine image is obtained by detecting gamma rays emitted from the radioactive medicine administered to a subject. However, gamma rays attenuate and scatter in a living body, and therefore, the intensity of the detected gamma rays increases or decreases depending on the depth from the body surface. Therefore, in reconstructing the nuclear medicine image from the intensity of the detected gamma rays, the intensity has to be corrected by taking into account the attenuation and scattering of the gamma rays in the living body. This correction is particularly important when the nuclear medicine image is used for quantitative evaluation of a living body.
In many cases, an attenuation or absorption map (μ map) that represents a distribution of the absorption or attenuation coefficient (μ value) in a target region is used. More specifically, the attenuation correction is carried out by estimating a distribution of scattered radiations from the μ map, correcting projection data by taking into account the scattered radiations, and then reconstructing an image using the μ map (see Hidehiro Iida et al., “Multicenter Evaluation of a Standardized Protocol for Rest and Acetazolamide Cerebral Blood Flow Assessment Using a Quantitative SPECT Reconstruction Program and Split-Dose 123I-Iodoamphetamine.”, J. Nucl. Med., (2010), vol. 51, No. 10, p. 1624-1631).